Rotary rock bits are used to penetrate earth formations in the drilling of oil and gas wells, geothermal steam wells, and related deep bores of accurately controlled diameter and direction. Typical modern rock bits have a main body with a threaded upper or pin end adapted for attachment to a drill pipe used in conventional rotary drilling. A plurality of conically shaped cutters or "cones" are rotatably mounted on legs extending downwardly from the main body, and the cones have teeth or hard inserts which contact and crush the rock formation being penetrated. Drilling mud is pumped through the drill pipe and passages in the bit body to cool the bit, and to flush cuttings from the bottom of the hole to the surface.
The main body of a typical modern bit is made by welding together several (usually three) mating segments. The lower end of each segment is a depending leg having a bearing pin at its lower end for rotatably supporting a rock-boring cutter cone. Each segment usually includes a pressure-compensated lubricant reservoir for supplying grease to the cone bearing.
In a typical three-cone bit, each segment has a pair of flat mating faces or faying surfaces oriented 120 degrees apart, and each face mates with a corresponding face on the adjacent segment. The assembly of three segments forms the main body of the bit, and the flat faces of adjacent segments lie in planes intersecting substantially along the bit axis. The segments are clamped in a holding fixture, checked for "gage" or diametral accuracy, and then welded together along the faying surfaces. Rock bits are subjected to extreme loads during drilling, and the segments must be secured together with a high-strength weld to avoid cracking.
A crack in the body of a rock bit can lead to leakage of drilling fluid from the interior of the bit to the exterior. Rock bits are often operated with abrasive drilling mud at pressures of over 1500 psi in the interior of the bit body. Leakage of drilling mud through a weld crack in the steel bit body can rapidly enlarge the crack to produce severe leakage. Such "wash-out" of the weld can change the flow direction of drilling fluid and degrade performance of the bit. Wear of the cones on the rock bit can be a significant problem if the mud-stream direction is not properly controlled. Pressure loss due to large leakage can result in premature pulling of the drill string with consequent loss of drilling time.
It has been common practice for many years to use arc welding to melt a filler metal into the interfaces between the segments to weld the segments together. This form of welding is expensive and time consuming, and requires a highly skilled operator if consistent results are to be achieved. A weld of this type does not always result in the strength which is desired in the welded assembly. Deep welds can require many passes to add enough filler metal to complete the weld. This method of welding also involves significant heating of the segments which may affect previous heat treatment of the welded parts and cause unpredictable warping of the completed bit outside of desired dimensional tolerances. Damage to resilient seals installed before welding can also be a problem arising from excessive heating.
Energy-beam welding is an alternate technique which has been perfected as a general welding method in recent years. The mating surfaces of the parts to be welded are irradiated with a focused beam of energy which melts the surfaces and forms a welded interface which may extend deeply between the parts in the direction of beam penetration. Laser beams and the like can be used as energy sources, but most commercial welding machines of this type use a high-energy electron beam which is generated in a vacuum chamber in which the parts to be welded are supported. The heat-affected zone lateral to the direction of the beam is shallow (minimizing warping and distortion of the parts), and substantially the entire faying surfaces of rock-bit segments can be welded to form a strong main body for the bit. Such welds are characterized by a depth of penetration much larger than the width of the heat-affected zone.
Use of energy-beam welding in the construction of rock-bit bodies is disclosed in U.S. Pat. Nos. 3,907,191 and 3,987,859 which discuss electron-beam welding methods in greater detail. These patents teach a procedure in which the bit segments are clamped together with mating faces in abutting contact without intervening spacers or shims. The segments are then electron-beam welded along the contacting surfaces of the mating faces.
We have found that a significant improvement in weld integrity and quality arises from placing a thin shim of an alloying metal such as titanium between the mating faces of the segments prior to welding with energy-beam techniques. Preferably, the shim is positioned at the central part of the bit-body dome or crown where the segments converge together, and shim thickness is limited to about 0.010 inch to avoid an excessive gap between the parts which could interfere with formation of a properly welded interface between the segments.
Use of steel shims between the segments of a rock-bit body has been known for many years as a dimensional control for assuring that bits assembled by gas or arc welding would have a correct gage diameter. Such shims were used when measurement showed that the gage diameter was out of tolerance. These shims, however, were isolated from the welded interface and did not affect the metallurgical properties of the weld.
A slurry of titanium powder in acetone has also been tried at one edge of the interface to be electron-beam welded. Such a slurry was applied to the surface of the body adjacent the Y-shaped intersection of the mating faces to be welded after the segments were assembled. The powder was placed on the dome surface and appeared to be scattered during electron-beam welding. It is believed that no more than a surface portion of the weld bead could be affected by the powder. No changes in weld properties were noted. So far as is known, shims of titanium or other alloying metal have not previously been positioned between the surfaces of steel rock-bit segments to be welded. Filler metal foil of aluminum alloy has been described for inhibiting cracking during welding of aluminum, and filler metal wire for electron-beam welding steel is also known for inhibiting cracking. See, for example, pages 530, 535, 536, 555 and 556 of Welding and Brazing, Metals Handbook, Vol. 6, 8th Edition, American Society for Metals (1971).
Another advantage of the use of shims is that the resulting opening of a slight gap between the segment faying surfaces in the region of the dome acts as a stress-relieving mechanism for the weld which minimizes residual stresses and the risk of cracking. The metal of the shim acts as a filler in critical regions to avoid depressions at the weld surface.
The shim metal alloys with the metal of the segments in the weld, thereby improving the weld ductility and crack resistance. When titanium or the like is used as the shim metal, it acts as a scavenger and stabilizer which deoxidizes the weld and stabilizes any excess carbon as titanium carbide to avoid embrittlement and cracking. The resulting weld is a fine-grain structure of improved ductility and impact strength, and excessive hardening of the weld during cooling is avoided.